Adolescence is a period of dramatic developmental transitions - from puberty-related changes in hormones, bodies, and brains to a complex psychological reorientation to the social world. The concurrent and marked increase in the onset of many psychiatric syndromes has prompted the search for key developmental processes that drive changes in risk for psychopathology during this period of life. Pubertal development is a prime candidate, but the precise neurobiological mechanisms via which puberty impacts adolescent-emergent mental health problems are still unknown. Hormonal surges associated with pubertal development in early adolescence are believed to impact a range of factors associated with brain development, psychological motivations, and social behavior that have demonstrated associations with risk for mental health symptoms. Therefore, this complex transition spanning molecules, neural circuits, and behavior is particularly well-suited for investigation using the RDoC mechanism. The proposed research focuses on constructs from the RDoC domain of Systems for Social Processes that change rapidly during adolescence: Self-Knowledge, Understanding of Mental States, and Affiliation. We hypothesize that these social processes (and their underlying neural circuitry) mediate the known relationship between the biological cascade of development associated with puberty and adolescent-emergent mental health problems. Accordingly, this proposal aims to conduct a comprehensive multilevel investigation of the connections between social cognitive and biological changes during early adolescence, in order to reveal the ways in which these interconnected changes relate to risk for the emergence of a range of mental health problems that are known to be associated with pubertal development (i.e., symptoms of depression, anxiety, and deliberate self-harm). Specifically, we will accomplish this goal by conducting a prospective longitudinal neuroimaging study of adolescent girls including three waves of data collection separated by 18 months (initial N=170, age 11 1 years). At each time point we will assess: i) levels of adrenal and gonadal hormones associated with pubertal development, as well as anthropometric data and secondary sex characteristics; ii) brain structure, anatomical connectivity (diffusion tensor imaging), and resting-state functional connectivity; iii) social cognitive brain functioning and behavior (in two paradigms measuring self-evaluation, perspective-taking, and intrinsic value of self-disclosure); and iv) mental health symptoms (particularly of depression, anxiety, and deliberate self-harm). We will use advanced statistical modeling techniques to describe the cross-sectional and longitudinal relationships between developmental patterns in each of these domains. The project will therefore inform a mechanistic model of the association between developmental and psychopathological processes during this stage of life, one that will critically inform early intervention and prevention strategies.